The overall purpose of these studies is to understand the cell cycle kinetic effects of radiation on slowly- or noncycling cells stimulated to proliferate rapidly. Two hypotheses will be tested: first, that accelerated proliferation of slowly cycling cells in response to injury is perturbed by radiation exposure; and second, that one such perturbation, a block or delay in the recruitment of quiescent cells, alters subsequent radiation-induced effects on cell cycle traverse and cell division during the "repopulation phase" of the proliferative response. Whether such kinetic effects will be productive, or counterproductive, with respect to radiotherapy, or other types of cancer treatment, depends on the existence of exploitable differences between normal tissues and tumors. The proliferative status of tissues is of considerable importance for the practice of radiation oncology insofar as cell kinetic state modulates cellular radiosensitivity and repair capacity within tumors and normal tissues, and governs. the onset and rate of repopulation in these tissues. These factors, in turn, influence the overall responsiveness of the tissue to radiotherapy, with implications both for normal tissue complications and tumor control. A cell culture model system consisting of confluent cultures of untransformed and transformed human breast epithelial cells will be developed, and time-dose relationships for cell cycle kinetic effects by Co-60 gamma-rays will be determined. These will be compared for different radiation schedules isoeffective for cell killing. The stimulus for cell proliferation will be provided by introducing "wounds" into the confluent cell layers. Cytochemical and immunocytochemical markers of cell cycle position will be used to monitor both recruitment from quiescence and the progression of individual cells around the remainder of the cell cycle in response to the proliferative stimulus. The cell cycle analysis will be conducted, and quantitation achieved, in situ, thus preserving any relevant positional information such as the relationship between cell cycle status and proximity to the wound. The sequential or simultaneous application of one or more of these cell cycle markers will provide a dynamic picture of the proliferative response in the untransformed and transformed monolayers, such that the total amount of "repopulation delay" for a given time-dose schedule can be further resolved into effects on individual cell cycle phase transitions. The proposed experiments may elucidate fundamental differences in proliferation kinetics between normal and cancer cells, and identify differences in the proliferative responses of rapidly, slowly or noncycling cells exploitable for cancer therapy.